Power converters are used to convert electric power from one form to another, for example, to convert direct current (DC) power to alternating current (AC) power. One important application for power converters is in transferring power from energy sources such as solar panels, batteries, fuel cells, etc., to electric power distribution systems such as local and regional power grids. Most power grids operate on AC current at a line (or mains) frequency of 50 or 60 cycles per second (Hertz or Hz). Power in a single phase AC grid flows in a pulsating manner with power peaks occurring at twice the line frequency, i.e., 100 Hz or 120 Hz. In contrast, many energy sources supply DC power in a steady manner. Therefore, a power conversion system for transferring power from a DC source to an AC grid typically includes some form of energy storage to balance the steady input power with the pulsating output power.
This can be better understood with reference to FIG. 1 which illustrates the mismatch between a DC power source and a 60 Hz AC load. As shown, the amount of power available from the DC source is constant (or could be varying slowly). However, the amount of power transferred to the load is of the form sine-squared which is the product of the sinusoidal load voltage, and the corresponding sinusoidal load current. As shown, the sine-squared load-power waveform fluctuates from the zero power level at the minimum of the sin-squared waveform to a maximum value and back down to minimum twice every line cycle. For a system with a grid frequency, f_grid, a cycle-time corresponding to twice the line frequency is given by 1/(2*f_grid), which is 10 millisecond (ms) for 50 Hz systems, and 8.33 ms for 60 Hz system. During time T1, the power available from the DC source exceeds the instantaneous power required by the AC load. During time T2, however, the maximum power available from the DC source is less than that required by the AC load.
FIG. 2 illustrates a conventional system for converting DC power from a photovoltaic (PV) panel to AC power. The PV panel 10 generates a DC output current IPV at a typical voltage VPV of about 35 volts, but panels having other output voltages may be used. A DC/DC converter 12 boosts VPV to a link voltage VDC of a few hundred volts. A DC/AC inverter 14 converts the DC link voltage to an AC output voltage VGRID. In this example, the output is assumed to be 120VAC at 60 Hz to facilitate connection to a local power grid, but other voltages and frequencies may be used.
The system of FIG. 2 also includes a DC link capacitor CDC and a decoupling capacitor C1. Either or both of these capacitors may perform an energy storage function to balance the nominally steady power flow from the PV panel with the fluctuating power requirements of the grid. Power ripple within the system originates at the DC/AC inverter 14, which must necessarily transfer power to the grid in the form of 120 Hz ripple. In the absence of a substantial energy storage device, this current ripple would be transferred all the way back to the PV panel where they would show up as fluctuations (or “ripple”) in the panel voltage VPV and/or current IPV. Therefore, the DC link capacitor CDC or the decoupling capacitor C1, is used to store enough energy on a cycle-by-cycle basis to reduce the ripple at the PV panel to an acceptable level.
In a relatively recent development, the ripple at the PV panel has been reduced to essentially zero through the use of one or more control loops that cause the DC/DC converter 12 to draw constant power from the PV panel while allowing the voltage on the link capacitor CDc to vary over a relatively wide range. See, e.g., U.S. Patent Application Publication Nos. 2010/0157638 and 2010/0157632 which are incorporated by reference.
A problem with prior art approaches, however, is that the power may be processed sequentially through multiple power stages. But each stage introduces various inefficiencies, so the overall system efficiency is reduced. Also, since each stage must be designed to carry the full system power, the components in each stage must be sized accordingly, which may increase the cost and reduce the reliability of the components.